1. Field of the Invention
The invention concerns a thin broadband microstrip array antenna.
The invention was made in the course of collaborative work with the electronics laboratory of the University of Nice Sophia-Antipolis, CNRS associated research team n.degree. 1400, on which Mr MONACO worked under the supervision of Messrs CAMBIAGGIO and PAPIERNIK.
2. Description of the Prior Art
A radio frequency electromagnetic wave characterized among other things by its wavelength .lambda., conveying energy and usually information, can propagate in various media the most important of which are:
guided propagation media (cables, lines, waveguides, etc.); and PA1 free space propagation media (homogeneous or non-homogeneous, isotropic or non-isotropic free space, etc.). PA1 the voltage standing wave ratio (VSWR) which allows for the quality of matching, i.e. the quantity of energy transmitted from the feed line to the antenna (the better this quality the closer the VSWR is to unity); PA1 the radiation diagram representing the spatial distribution of the electromagnetic field E of the wave; and PA1 associated conventional parameters (gain, directivity, efficiency, -3 dB aperture, coverage probability). PA1 either guide an electromagnetic wave (microstrip line); PA1 or radiate an electromagnetic field (microstrip antenna). PA1 either the air-substrate interface; PA1 or the air-conductor-substrate interface. PA1 the radiation diagram is then largely predictable; and PA1 the sizing of these elements to resonate at a given frequency is well understood. PA1 .lambda..sub.o is the wavelength in air at the frequency F.sub.r (i.e. the speed of light divided by this frequency), and PA1 W is the width of the patch, according to the above work, for example, defined by the equation: ##EQU4## PA1 serial, PA1 parallel, PA1 combination of serial and parallel. PA1 thin, PA1 light in weight, PA1 of low cost (quick and easy to manufacture), and PA1 able to be "conformed" to apply them to, for example, cylindrical and conical structures. PA1 small increase in bandwidth; PA1 increased ohmic losses in the substrate; PA1 generation of surface waves, and PA1 increased antenna overall size. PA1 increased thickness, which may be unacceptable if the antenna is required to be thin, especially if it must be conformed (aerospace applications, launch vehicles); PA1 mechanical inconsistencies and discontinuities affecting the performance of the antenna if it is subject to mechanical or thermal stress (antenna on aircraft, missiles or satellites); and PA1 problems in manufacturing the antenna respecting the dimensions and relative positions of the various layers (affecting the radio frequency performance). PA1 the complexity of the feed network; and PA1 the large overall size of the "plural-radiators" which is incompatible with severe constraints in respect of the maximal spacing between elements active at a given time usually imposed for controlling the radiation diagram (for example: .DELTA./.lambda..sub.o &gt;0.5). PA1 compliance with severe spacing constraints (.DELTA./.lambda..sub.o &lt;1, or even .DELTA./.lambda..sub.o &lt;0.5) between the phase centers imposed for reasons of small overall size or for better control of the radiation diagram, preferably combined with compact transverse dimensions; PA1 a greater bandwidth than with patches that are all identical; PA1 small overall thickness (in particular, thin dielectric); PA1 feasibility of single-layer (i.e. single layer of dielectric) and multi-layer structures; PA1 possibility of using a "coplanar" feed network, i.e. a network on the same side of the dielectric as the patches; PA1 possibility of conforming the antenna; and PA1 high mechanical strength. PA1 the parasitic patches are either of a first type having a first resonant frequency or of a second type having a second resonant frequency whereby the antenna is formed by the repetition of groups of four patches. PA1 The parasitic patches are of a single type having a common resonant frequency near the resonant frequency of the active patches; PA1 At least some of the parasitic patches are offset relative to the active patches transversely to the alignment direction. PA1 The parasitic patches are offset in the same direction. PA1 The distance .DELTA. between the phase centers of the active patches is at most in the order of .lambda..sub.o /2; PA1 The active patches are connected in parallel to a feed circuit. PA1 The width of the slots is at most equal to .lambda..sub.o /(50..sqroot.(.lambda..sub.r +1)/2) where .lambda..sub.r is the dielectric constant of the dielectric support; PA1 The constant width slots extend over at least 80% of the transverse dimension of the active patches.
An antenna may be regarded as an interface between these two types of media enabling partial or total transfer of electromagnetic energy from one to the other. A transmit antenna passes this energy from a guided propagation medium to a free space propagation medium and a receive antenna reverses the direction of energy transfer between the media. The following description usually refers implicitly to a transmit antenna. However, the principle of equivalence guarantees reciprocity of all stated properties with a receive antenna.
The expression antenna feed circuit(s) or device refers to all component parts of all or part of the guided propagation medium directing or collecting the electromagnetic energy to be transferred and comprising passive or active, reciprocal or non-reciprocal components.
An elementary antenna is often associated with one or more geometrical points called phase centers from which the electromagnetic wave appears to emanate for a given direction in the case of a transmit antenna.
Antenna resonance occurs at the frequency or frequencies at which the transfer of energy transmitted from the feed line to free space via the antenna is optimum; in mathematical terms, at the resonant frequency F.sub.r the complex impedance Z at the antenna input has a null imaginary part and a maximal real part.
In microwave technology it is usual to represent the locus of the impedances Z (as a function of frequency) on a SMITH chart on which each resonance appears as a loop.
Using current measuring techniques this resonance is "seen" through the matching arrangement which characterizes the transfer of energy from the feed line to the antenna. This view of the antenna behavior may be called the antenna response and is quantified in terms of return losses or the voltage standing wave ratio (VSWR) as defined below.
If Z is the impedance at the point at which matching is measured and Z.sub.c is the characteristic impedance of the feed line (according to the standard usually adopted Z.sub.c =50 Ohms), then if z=Z/Z.sub.c the return loss is the complex ratio: EQU .rho.=(z-1)/(z+1)
The VSWR is then defined as: EQU VSWR=.vertline.(1+.vertline..rho..vertline.)/(1-.vertline..rho..vertline.). vertline.
The antenna is characterized by a number of performance indicators including:
The radiation diagram is conventionally represented in a frame of reference centered at a point on the antenna (its phase center if possible) and shown as "cross-sections" in a standardized system of spherical coordinates (.theta., .phi.). A so-called "constant .phi." cross-section is the curve of variation in the field E projected onto a given polarization (either E.theta. or E.phi.), .theta. varying from 0.degree. to 180.degree. (or from -180.degree. to +180.degree.). Likewise, a so-called "constant .theta." cross-section is the curve of variation in the field E projected onto a given polarization (either E.theta. or E.phi.) with .phi. varying from 0 to 360.degree..
An association of elementary antennas is called an antenna array if their feed circuits have common parts or if, because of coupling between the elementary antennas, the overall radiation diagram of the array in a given frequency range depends on that of each of the antennas or radiating elements.
The array obtained by the arrangement of antennas similar to one or more elementary antennas on a given surface is often called an array antenna, usually implying a concept of geometrical repetition of the elementary antennas.
Array antennas are usually employed to obtain a radiation diagram that is highly directive in a given direction relative to the array.
The spacing .DELTA. between the phase centers of the elementary antennas of the array divided by the wavelength .lambda..sub.o in air or in vacuum is often a critical parameter.
For example, for values of .DELTA./.lambda..sub.o &gt;0.5 the occurrence of significant grating lobes outside the wanted radiation area penalizes the energy transmission balance in the free space propagation medium.
The microstrip technology entails stacking a plurality of layers of conductive or dielectric materials such as, for example, a dielectric substrate layer (glass, PTFE, for example) coated on its lower surface (or I surface) with a conductive film (copper, gold, etc.) known as the ground plane and carrying on its upper surface (or S surface) a discontinuous conductive film forming a given geometrical pattern made up of what are usually called patches.
This system can:
The medium in which surface currents propagate is:
In the former case the "effective" dielectric constant of the medium may be defined as: ##EQU1## where .epsilon..sub.r is the dielectric constant of the substrate (cf MICROSTRIP ANTENNAS by I. J. BAHL and P. BHARTIA, ARTECH HOUSE, 1980).
In the second case: ##EQU2## where h is the substrate thickness and w is the width of the conductor strip.
Various types of (possibly active) components or other elements may usually be provided on the S side of the structure.
By definition a microstrip antenna is a geometrically shaped element of conductive material on the S side of a dielectric layer. A rectangular or circular shape is often chosen for the following reasons:
A rectangular microstrip patch is to some extent similar to two parallel slots coincident with two radiating edges of the rectangle. The edges of a rectangular patch which must radiate (and conversely those which must not radiate) are selected by an appropriate choice of the part of the rectangle which is connected to the feed circuit. A rectangular patch is usually fed near or on the median line joining the sides to be made to radiate.
This connection may be made through the dielectric substrate or at the periphery of the patch by a microstrip line on the S side (the expression coplanar feed is sometimes used) as described in French Patent 2,226,760, among others.
It is essentially the length L of the patch between these edges which determines the antenna resonant frequency.
Appropriate equations and nomograms have been produced.
In MICROSTRIP ANTENNAS by I. J. BAHL and P. BHARTIA, ARTECH HOUSE, 1980, it is stated that to resonate at the frequency F.sub.r a rectangular patch must have a length L such that: ##EQU3## .epsilon..sub.e is the dielectric constant of the dielectric substrate, h is the height (or thickness) of the substrate,
The choice of the width W conditions to a large degree the quality of the radiation, i.e. its efficiency and its form.
The above work also indicates that the radius R of a circular patch is given by the equation: ##EQU5## where: EQU K=8.794/(f.sub.r .sqroot..epsilon..sub.r ) (5)
Any microstrip patch may be used as an element of an array of the following types:
This technology produces antennas (or antenna arrays) that are:
The microstrip antenna is an electronic resonator which is designed to have a high Q. Because of this, antennas using this technology always have a small bandwidth, i.e. resonance occurs in a localized manner only at the frequency for which the antenna is sized and at frequencies very near this frequency.
For example, a conventional rectangular microstrip antenna sized to resonate at 1.6 GHz on a 1 mm thick substrate with dielectric constant .epsilon.r=2.2 is usable only in a frequency band whose width is in the order of 1% of the resonant frequency, which is insufficient for most applications (telemetry, etc).
Various methods have previously been proposed to overcome this problem.
The simplest way to increase the antenna bandwidth is to make the dielectric layer thicker. This method has the following drawbacks:
The concept most often used is to stack radiating elements that are not fed (with their associated dielectric layer) on the fed element. These elements are called "parasitic elements". Each of these elements i is sized to resonate at a frequency F.sub.i near the frequency F.sub.a of the fed element. Electromagnetic coupling between these elements and the fed elements causes transfer of energy to the "parasitic elements". The overall frequency response is the envelope of the responses of each element.
This so-called multilayer structure and structures derived from it have the following disadvantages:
Consideration might be given to including a layer of air (an excellent dielectric) between the I and S surfaces. This concept is merely a variant of what is described above and has the same mechanical drawbacks.
There thus remains in some applications the requirement to develop a single layer (i.e. only one dielectric layer) structure broadband antenna which avoids the above drawbacks.
It has already been proposed to place two rectangular parasitic patches along non-radiating sides of a fed rectangular patch, or even four rectangular parasitic patches along sides of this patch, in order to enable strong coupling between the facing sides of these patches. Reference may be had to the document WO-89/07838 or to the article "Non-radiating Edges and Four Edges Gap-Coupled Multiple Resonator Broad Band Microstrip Antennas" by G. KUMAR and K. C. GUPTA published in I.E.E.E. Transactions on Antennas and Propagation, Vol. AP 33 n.degree. 2, February 1985. There are preferably four parasitic patches whose dimensions are at least similar to the central patch.
An array of such antennas is obtained by reproducing periodically along one or even two directions in a plane groups of three (or preferably five) patches of which only one is fed, which raises problems of overall size since between two fed patches there are two parasitic patches separated by a substantial gap; also, the feed is in principle from under the surface carrying the patches (see in particular the document WO-89/07838 which is the only one of the aforementioned two documents to make express provision for producing an array of this kind). The disclosures of these two documents are therefore unable to meet a spacing constraint in the nature of .DELTA.&lt;0.5 .lambda..sub.o, for example.
British Patent 2,067,842 also discloses the association on a dielectric substrate of at least one parasitic patch near a rectangular patch fed through the substrate, of the same size and grounded; either the fed patch and the single parasitic patch are both grounded via their opposite sides or the fed patch is isolated and surrounded with two or even four parasitic patches coupled to ground by their sides adjacent the fed central patch. The above drawbacks, among others, are encountered.
U.S. Pat. No. 4,933,680 proposes an array whose constituent elements are not simple patches but a plurality of elementary patches ("plural-radiators") resonating at different frequencies (and therefore having different geometries) which may be separated by parasitic patches. These elementary patches are not fed simultaneously. At each operating frequency a set of circulators exploits the mismatching of non-resonant elements to "route" the maximum energy from the feed network to the element resonating at the required operating frequency.
This structure enables operation in a given frequency band with a single-layer structure but has drawbacks which render it unusable in some applications:
An object of the invention is to alleviate the aforementioned drawbacks by virtue of an array antenna (i.e. an antenna formed by a periodic array of elementary patches) combining advantages including: